Figure 1: Artist's impression of an exoplanet.
By visually inspecting the light curves of 7557 Kepler Objects of Interest (KOIs) to search for single transit events (STEs) caused by giant planets on long-period orbits, Uehara et al. (2016) present the detection of seven transiting planet candidates that are consistent with Neptune-sized to Jupiter-sized planets with orbital periods ranging from a few to ~20 years.
These 7 planet candidates orbit their host stars beyond the snowline. The snowline is basically the distance from a star where temperatures in the protoplanetary disk start to become cold enough for volatiles such as water to condense into solid grains.
Of the 7 KOIs in this study, 2 are host to compact multi-transiting planetary systems. This suggests that over 20 percent of compact multi-transiting planetary systems also host cool gaint planets with orbital periods exceeding 3 years. The 7 KOIs in this study are KOI-847, KOI-671, KOI-2525, KOI-1108, KOI-693, KOI-435 and KOI-1421.
Figure 2: Artist's impression of an exoplanet.
KOI-847 is a planetary system with a known Neptune-sized planet candidate whose orbital period is 80.9 days. Two STEs were found which indicate the presence of two Neptune-sized planet candidates whose orbital periods are ~840 days and ~930 days. Since the two transits are nearly identical, they are likely to be caused by just one Neptune-sized planet candidate rather than two. In this case, the orbital period of the Neptune-sized planet candidate is ~1106 days.
KOI-671 is a known compact multi-transiting planetary system with four transiting planet candidates orbiting their host star closer than Mercury is to the Sun. The STE observed for this system indicates the presence of a Neptune-sized planet candidate with 3.9 ± 2.5 times the radius of Earth and a remarkably long orbital period of roughly 7700 days (~20 years). If confirmed, this should be the longest period transiting planet found to date.
KOI-2525 is known to host one super-Earth candidate. The STE observed for this planetary system indicates the presence of a second planet candidate with 12.7 ± 1.3 times the radius of Jupiter in a ~1200 day orbit around its host star.
KOI-1108 is a planetary system with three known transiting super-Earth candidates in close-in orbits around the host star. The STE observed for KOI-1108 corresponds to a Neptune-sized planet with 5.5 ± 1.9 times the radius of Earth and an orbital period of roughly 1160 days.
KOI-693 is a planetary system hosting two confirmed super-Earths. The STE observed for KOI-693 is consistent with a Neptune-sized planet with 3.5 ± 1.5 times the radius of Earth and an orbital period of roughly 980 days.
KOI-435 is a planetary system with two confirmed planets and three planet candidates. All five planets and planet candidates are in the super-Earth/sub-Neptune regime. The STE observed for this planetary system corresponds to a planet candidate with 7.8 ± 3.3 times the radius of Earth in a ~910 day orbit around its host star.
KOI-1241 only has a STE observed for it. The STE corresponds to a planet candidate with 10.2 ± 4.5 times the radius of Earth, in a ~2230 day orbit around its host star.
Figure 3: Artist's impression of an exoplanet.
Uehara et al. (2016), "Transiting Planet Candidates Beyond the Snow Line Detected by Visual Inspection of 7557 Kepler Objects of Interest", arXiv:1602.07848 [astro-ph.EP]